The increasing interest in developing ocean technologies requires a more thorough understanding of the ocean's properties and its characteristics. Recently, because of a number of inherent advantages, the optical transmission of information through the ocean medium has received a good deal of attention. However, several factors must be taken into consideration. Although a variety of factors influence the quality and reliability of such communications, one that is of particular interest is knowing the diffuse attenuation coefficient K. This coefficient is used to describe the attenuation of downwelling irradiance as a function of depth in natural waters and is an apparent optical property. It is a function of the geometry of measurement and other factors which alter the irradiance distribution as a function of depth.
A thorough explanation and analysis of the diffuse attenuation coefficient parameter is covered in an article by K. S. Baker and R. C. Smith entitled "Quasi-Inherent Characteristics of the Diffuse Attenuation Coefficient for Irradiance" appearing in the Ocean Optics No. 4 issue of the Society of the Photo-Optical Instrumentation Engineers (1979). Definitions, applications and a somewhat extensive idealized data gathering scheme are dealt with in the article and limitations in the approach are listed.
Since the diffuse attenuation coefficient is an apparent optical property, its behavior with depth exhibits reproducible regularities in a wide range of natural water types and it is possible to formulate exact mathematical interrelationships between it and the inherent optical properties. As a consequence, knowing the diffuse attenuation coefficient permits practical solutions to a wide range of problems in ocean optics.
The data obtained by Baker and Smith was gathered under almost ideal environmental and experimental conditions. First, the atmospheric conditions had to remain uniform throughout the data gathering period (all day) and, secondly, the air-water interface had to be relatively smooth for waves increase the uncertainty in measuring depths accurately and require longer integration times to obtain average irradiance values at each depth; the latter is an important factor if complete spectral and depth data are to be obtained throughout the day as a function of the sun angle to the surface. Third, the water column had to be relatively uniform and remain so throughout the duration of the experiment. And lastly, favorable experimental conditions had to be maintained all day to allow accurate data gathering, that is, all instrumentation must remain functional on a moored barge positioned on station. Although the barge provided a stable platform for rapid, efficient and continuous monitoring by several instruments, its shadow compromised the measurements at high noon.
From the foregoing it is apparent that determination of the diffuse attenuation coefficient depends on sea state, wind state, atmospheric conditions, a stable support platform (nonshadowing) as well as availability of numerous instruments and trained operating personnel. Ideal environmental and experimental conditions must coincide. Unfortunately, this highly desirable situation is not as frequent as experimenters would like.
Thus, there is a continuing need in the state-of-the-art for a diffuse attenuation coefficient measuring device which is capable of reliably providing the needed information under a variety of conditions and during the daytime and nighttime.